plain¹ / plān/ • adj. 1. not decorated or elaborate; simple or ordinary in character: good plain food everyone dined at a plain wooden table. ∎  without a pattern; in only one color: a plain fabric. ∎  bearing no indication as to source, contents, or affiliation: donations can be put in a plain envelope. ∎  (of a person) having no pretensions; not remarkable or special: a plain, honest man with no nonsense about him. ∎  (of a person) without a special title or status: for years he was just plain Bill.2. easy to perceive or understand; clear: the advantages were plain to see it was plain that something was very wrong. ∎  (of written or spoken usage) clearly expressed, without the use of technical or abstruse terms: written in plain English. ∎  not using concealment or deception; frank: he recalled her plain speaking.3. (of a person) not beautiful or attractive: the dark-haired, rather plain woman.4. sheer; simple (used for emphasis): the main problem is just plain exhaustion.• adv. inf. clearly; unequivocally (used for emphasis): perhaps the youth was just plain stupid.• n. a large area of flat land with few trees. Compare with prairie. ∎  (the Plains) another term for Great Plains.PHRASES: as plain as the nose on one's face inf. very obvious.plain and simple inf. used to emphasize the statement preceding or following: she was a genius, plain and simple.plain as day inf. very clearly.DERIVATIVES: plain·ness n.plain² • v. [intr.] archaic mourn; lament. ∎  complain. ∎  emit a mournful or plaintive sound.

PLAIN. A term for direct and unambiguous language: ‘Speketh so pleyn at this tyme, I yow preye, / That we may understonde what ye seye’ ( Chaucer, the Host speaking to the Clerk of Oxford in The Canterbury Tales); The Complete Plain Words (title of a British usage manual by Sir Ernest Gowers, 1954). Plain has been used since the late Middle Ages to contrast with ornate, academic, technical, Latinate, etc., and phrases such as plain English, plain language, plain style have been used to contrast with such expressions as double Dutch, gobbledygook, jargon, and mumbo jumbo. The contrast of plain style with other styles of language originates in ancient Greece and Rome, when RHETORIC recognized three styles: the grand or high style, the middle style, and the plain or low style, each appropriate to certain audiences, occasions, and purposes. The grand declamatory style was suited to high themes, people, and occasions (with the risk of becoming grandiose), the moderate middle style was suited to instruction and education (with the risk of becoming bland or pedantic), and the simple style was suited to ordinary life and public speaking (with the risk of becoming coarse, colourless, or patronizing). During the Renaissance and Reformation (15–16c), styles tended to polarize between the classical, academic, and ornate on the one side and the popular, vernacular, and plain on the other, a division that mirrored religious disputes. Protestants, especially of the ‘Low Church’ in England, favoured Puritan plain style and Quaker plain language in their worship, speech, and writing. Both forms influenced the development of prose writing in the British Isles, North America, and elsewhere. See ANGLO-SAXON, COMPLETE PLAIN WORDS, SAXONISM.

plain simple, uncomplicated.
plain as a pikestaff very plain. The phrase was originally (in the mid 16th century) plain as a packstaff, a packstaff being the staff on which a pedlar supported his wares while resting.
plain living and high thinking denoting a frugal and philosophic lifestyle; the original allusion is to Wordsworth's lines ‘Plain living and high thinking are no more: The homely beauty of the good old cause Is gone.’
Plain People the Amish, the Mennonites, and the Dunkers, three strict Christian sects emphasizing a simple way of life.
plain sailing used to describe a process or activity which goes well and is easy and uncomplicated. The phrase, which is mid 18th century, is probably a popular use of plane sailing, denoting the practice of determining a ship's position on the theory that it is moving on a plane.

plain clear, manifest XIII; †flat, level, even; unembellished; free from duplicity or ambiguity XIV; ordinary, simple XVI. — OF. plain, fem. -e :— L. plānus, -a, f. base *plā- flat, of obscure orig.
Hence plainly XIV. So plain sb. flat tract of country. XIII. OF. plain (superseded by plaine :— L. coll. n. pl.) :— L. plānum, sb. use of n. of adj.. See also PLANE3, PLANE4.